An organic electroluminescent device (hereinafter often called organic EL device) is a spontaneously luminous device which features higher brightness and higher legibility than those of the liquid crystal devices enabling vivid display to be attained and has, therefore, been vigorously studied.
In 1987, C. W. Tang et al. of the Eastman Kodak Co. have developed a device of a layer-laminated structure comprising various kinds of materials to bear individual roles, and have put an organic EL device using organic materials into a practical use. The above organic EL device is constituted by laminating layers of a fluorescent body capable of transporting electrons and an organic material capable of transporting holes. Upon injecting both electric charges into the layer of the fluorescent body to emit light, the device is capable of attaining a brightness of as high as 1000 cd/m2 or more with a voltage of not higher than 10 V.
So far, very many improvements have been made to put the organic EL device to practical use. For example, the organic EL device has been widely known having a structure comprising an anode, a hole injection layer, a hole-transporting layer, a luminous layer, an electron-transporting layer, an electron injection layer and a cathode which are arranged in this order on a substrate more finely dividing their roles than ever before. The device of this kind is achieving a high efficiency and a high durability.
To further improve the luminous efficiency, attempts have been made to utilize triplet excitons and study has been forwarded to utilize a phosphorescent luminous compound.
In the organic EL device, the electric charges injected from the two electrodes recombine together in the luminous layer to emit light. Here, however, the hole migration rate is higher than the electron migration rate arousing a problem of a decrease in the efficiency since the holes partly pass through the luminous layer. Therefore, it has been desired to provide an electron-transporting material that has a higher electron migration rate.
Tris(8-hydroxyquinoline)aluminum (hereinafter abbreviated as Alq3) which is a representative luminous material has also been generally used as an electron-transporting material having, however, a low electron mobility and a work function of 5.6 eV and, therefore, having a hole-blocking capability which is far from satisfactory.
A method of inserting a hole-blocking layer is one of the measures for preventing the holes from partly passing through the luminous layer to improve the probability of recombination of the electric charge in the luminous layer.
As a hole-blocking material used for forming the hole-blocking layer, for example, a patent document 1 discloses a 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (hereinafter abbreviated as TAZ).
As the hole-blocking material, there have, further, been known a bathocuproin (hereinafter abbreviated as BCP) and a mixed ligand complex of aluminum such as aluminum (III) bis(2-methyl-8-quinolinato)-4-phenyl phenolate (hereinafter abbreviated as BAlq).
The TAZ has a work function of as large as 6.6 eV and a large hole-blocking power, and is used for forming an electron-transporting hole-blocking layer that is laminated on the cathode side of a fluorescent luminous layer or a phosphorescent luminous layer prepared by vacuum evaporation or by coating and, therefore, contributes to improving the efficiency of the organic EL devices.
Because of its low electron-transporting capability, however, the TAZ had to be used in combination with an electron-transporting material having a higher electron-transporting capability. The BCP, on the other hand, has a work function of as large as 6.7 eV and a large hole-blocking power but a glass transition point (Tg) of as low as 83° C. In the form of a thin film, therefore, the BCP lacks stability and still leaves much room for improvement for forming a hole-blocking layer that works maintaining stability.
A patent document 2 discloses a general electron-transporting compound which, however, still lacks stability when it is formed into a film or lacks the function for blocking the holes to a sufficient degree.
In order to improve characteristics of the organic electroluminescent devices, therefore, it has been desired to provide an organic compound that excels in electron injection/transporting capability and in hole-blocking power, and features high stability in the form of a thin film.